Object detection neural networks

ABSTRACT

Methods, systems, and apparatus, including computer programs encoded on computer storage media, for generating object detection predictions from a neural network. In some implementations, an input characterizing a first region of an environment is obtained. The input includes a projected laser image generated from a three-dimensional laser sensor reading of the first region, a camera image patch generated from a camera image of the first region, and a feature vector of features characterizing the first region. The input is processed using a high precision object detection neural network to generate a respective object score for each object category in a first set of one or more object categories. Each object score represents a respective likelihood that an object belonging to the object category is located in the first region of the environment.

FIELD

This specification relates to autonomous vehicles.

BACKGROUND

Autonomous vehicles include self-driving cars, boats, and aircraft.Autonomous vehicles use a variety of on-board sensors and computersystems to detect nearby objects and use such detections to make controland navigation decisions.

Some autonomous vehicles have computer systems that implement neuralnetworks for object classification within images.

Neural networks, or for brevity, networks, are machine learning modelsthat employ multiple layers of operations to predict one or more outputsfrom one or more inputs. Neural networks typically include one or morehidden layers situated between an input layer and an output layer. Theoutput of each layer is used as input to another layer in the network,e.g., the next hidden layer or the output layer.

Each layer of a neural network specifies one or more transformationoperations to be performed on input to the layer. Some neural networklayers have operations that are referred to as neurons. Each neuronreceives one or more inputs and generates an output that is received byanother neural network layer. Often, each neuron receives inputs fromother neurons, and each neuron provides an output to one or more otherneurons.

An architecture of a neural network specifies what layers are includedin the network and their properties, as well as how the neurons of eachlayer of the network are connected. In other words, the architecturespecifies which layers provide their output as input to which otherlayers and how the output is provided.

The transformation operations of each layer are performed by computershaving installed software modules that implement the transformationoperations. Thus, a layer being described as performing operations meansthat the computers implementing the transformation operations of thelayer perform the operations.

Each layer generates one or more outputs using the current values of aset of parameters for the layer. Training the network thus involvescontinually performing a forward pass on the input, computing gradientvalues, and updating the current values for the set of parameters foreach layer. Once a neural network is trained, the final set ofparameters can be used to make predictions in a production system.

Convolutional neural networks include convolutional neural networklayers. Convolutional neural network layers have a neuron connectivitythat takes advantage of spatially local correlation in the input data.To do so, convolutional neural network layers have sparse connectivity,with neurons in one convolutional layer receiving input from only asmall subset of neurons in the previous neural network layer. The otherneurons from which a neuron receives its input defines a receptive fieldfor that neuron.

Convolutional neural network layers have one or more filters, which aredefined by parameters of the layer. A convolutional neural network layergenerates an output by performing a convolution of each neuron's filterwith the layer's input.

In addition, each convolutional network layer can have neurons in athree-dimensional arrangement, with depth, width, and height dimensions.The width and height dimensions correspond to the two-dimensionalfeatures of the layer's input. The depth-dimension includes one or moredepth sublayers of neurons. Generally, convolutional neural networksemploy weight sharing so that all neurons in a depth sublayer have thesame weights. This provides for translation invariance when detectingfeatures in the input.

Convolutional neural networks can also include fully-connected layersand other kinds of layers. Neurons in fully-connected layers receiveinput from each neuron in the previous neural network layer.

Autonomous and semi-autonomous vehicle systems can use object detectionpredictions for making driving decisions.

Autonomous vehicle systems can make object detection predictions usinghuman-programmed logic. The human-programmed logic specifies preciselyhow the outputs of on-board sensors should be combined, transformed, andweighted, in order to compute a full-vehicle prediction.

SUMMARY

In some implementations, an autonomous or semi-autonomous vehicle iscapable of using a high precision object detection neural network toautomatically identify and classify objects of interest in anenvironment around the vehicle. Examples of objects include pedestrians,cyclists, or other vehicles. Laser and camera sensors of the vehicleinitially capture a top-down projected laser image and a camera image,respectively, to characterize the environment surrounding the vehicle.The sensor data is pre-processed and provided as input to the objectdetection neutral network to identify candidate regions in theenvironment that satisfy a set of criteria associated with objectdetection. The candidate regions are then analyzed to identify and/orverify predicted objects within the candidate regions. The output of theobject detection neural network can include a list of detected objectsand for each detected object, respective object scores reflectinglikelihoods that the detected object belongs to the object categories.

Particular embodiments of the subject matter described in thisspecification can be implemented so as to realize one or more of thefollowing advantages. An autonomous or semi-autonomous vehicle systemcan use a fully-trained neural network subsystem to classify objectscorresponding to pedestrians or cyclists. The system can use techniquesto improve the accuracy of the predictions. The system can also processsensor data through specific neural network subsystems to reducecomputational resources required to generate accurate objectpredictions. For example, confidence scores of object scores forpredicted objects using a lower precision neural network can be used todetermine if additional processing is required. In response determiningthat the values of the confidence scores satisfy a threshold value, ahigher-precision neural network can be used to further process sensordata to improve object prediction accuracy.

The system can also detect objects with both “high precision” and “highrecall” phases that are efficient enough to be executed in “real time”for the purposes of automatic driving. For instance, as described below,the system uses a neural network architecture that includes a “highrecall” phase that initially generates object candidates, and a “highprecision” phase that then uses more accurate (e.g., higher precisionand higher recall) classifiers to generate a high precision objectdetection result with minimal loss of recall. In the high recall phase,the system processes sensor data quickly with lower accuracy, while inthe high precision phase, the system processes the output of the highrecall phase at a slower speed but with higher accuracy. The neuralnetworks described throughout can be trained end-to-end to produceeffective performance.

In addition, the system can be applied to process data from differentkinds of sensors, e.g., LADAR, and video cameras, and can combine thedata from different sensors to improve overall object detectionperformance.

In one general aspect, a method includes: obtaining an inputcharacterizing a first region of an environment. The input includes: (i)a projected laser image generated from a three-dimensional laser sensorreading of the first region; (ii) a camera image patch generated from acamera image of the first region; and (iii) a feature vector of featurescharacterizing the first region. The method further includes processingthe input using a high precision object detection neural network togenerate a respective object score for each object category in a firstset of one or more object categories. Each object score represents arespective likelihood that an object belonging to the object category islocated in the first region of the environment. Processing the inputfurther includes: processing the projected laser image through a lasersub-neural network to generate an alternative representation of theprojected laser image; processing the camera image patch through acamera sub-neural network to generate an alternative representation ofthe camera image patch; processing the feature vector through a featuresub-neural network to generate an alternative representation of thefeature vector; and processing the alternative representation of theprojected laser image, the alternative representation of the cameraimage patch, and the alternative representation of the feature vectorthrough a combining sub-neural network to generate the respective objectscore for each of the one or more object categories.

One or more implementations may include the following optional features.For example, in some implementations, the environment is an environmentin proximity of an autonomous vehicle. The three-dimensional lasersensor reading and the camera image are captured by sensors of theautonomous vehicle.

In some implementations, the combining sub-neural network is configuredto: process the alternative representation of the laser projected imageand the alternative representation of the camera image patch through oneor more visual combining neural network layers to generate a visualcombined representation; process the visual combined representation andthe alternative representation of the feature vector through one or morefinal combining layers to generate a final combined representation; andprocess the final combined representation through an output neuralnetwork layer to generate the object scores.

In some implementations, the method further includes: obtaining a cameraimage of the environment; and processing the camera image using a firsthigh-recall object detection neural network. The first object detectionneural network is configured to: receive the camera image; and processthe camera image to generate: (i) data defining a plurality of boundingboxes in the camera image; and (ii) for each of the plurality ofbounding boxes, a respective first confidence score that represents alikelihood that an object belonging to an object category from a secondset of one or more object categories is present in the region of theenvironment shown in the bounding box.

In some implementations, the method further includes: generating aplurality of camera image patches from the obtained camera image of theenvironment; determining a respective size value for each of theplurality of camera image patches; adjusting the respective size valuesfor each of the plurality of camera image patches to generate aplurality of range-normalized camera image patches, where the values ofthe respective sizes for each of the plurality of range-normalizedcamera image patches are substantially similar; and processing theplurality of range-normalized camera image patches using the firsthigh-recall object detection neural network.

In some implementations, the method further includes: obtaining aprojected laser image of the environment; processing the projected laserimage using a second high-recall object detection neural network. Thesecond high-recall object detection neural network is configured to:receive the projected laser image; and process the projected laser imageto generate: (i) data defining a plurality of bounding boxes in theprojected laser image; and (ii) for each of the plurality of boundingboxes, a respective second confidence score that represents a likelihoodthat an object belonging to an object category from the second set ofone or more object categories is present in the region of theenvironment shown in the bounding box.

In some implementations, the method further includes: obtainingrespective top-down projections of a plurality of channels of the inputcharacterizing the first region of the environment, each top-downprojection representing a projection of an area surrounding theautonomous vehicle; and combining the respective top-down projections togenerate the projected laser image.

In some implementations, the method further includes: determining, fromthe first confidence scores and the second confidence scores, thatadditional processing is required to determine a higher-precisionconfidence score for the first region, and in response, obtaining thefirst input characterizing the first region and processing the firstinput using the high precision object detection neural network.

In some implementations, the projected laser image is generated from athree-dimensional laser sensor reading collected by a long-range lasersensor.

In some implementations, the projected laser image is generated from athree-dimensional laser sensor reading collected by a short-range lasersensor.

In some implementations, the one or more object categories include atleast a pedestrian category and a cyclist category.

In some implementations, the obtained input characterizing the firstregion of the environment comprises a multiplexed signal, and processingthe input using a high precision object detection neural networkcomprises processing each subcomponent of the obtained input to generatea respective object score for a particular object category, where eachsubcomponent of the obtained input corresponds to a different objectcategory.

The details of one or more embodiments of the subject matter of thisspecification are set forth in the accompanying drawings and thedescription below. Other features, aspects, and advantages of thesubject matter will become apparent from the description, the drawings,and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an example of a system.

FIG. 2 is a flow chart of an example of a process for performing highprecision object detection for an environment in proximity of anautonomous vehicle.

FIG. 3A is a schematic diagram of an example of an architecture of ahigh precision object detection neural network.

FIG. 3B is a schematic diagram of an example of an architecture of ahigh precision object detection neural network that processes long-rangeand short-range sensor data.

FIG. 4A is a schematic diagram of an example of processing a projectedlaser image of an environment to generate bounding boxes that representdetected objects in the projected laser image.

FIG. 4B illustrates an example of processing a camera image of anenvironment to generate bounding boxes that represent detected objectsin the projected laser image.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

This specification describes how a vehicle, e.g., an autonomous orsemi-autonomous vehicle, can use one or more fully-learned neuralnetworks to automatically detect objects of certain object categories inan environment surrounding the vehicle. Each object category can specifyan object type that is commonly within a vicinity of the vehicle as ittravels on a road. For example, object categories can representpedestrians, cyclists, or other vehicles within a proximity to thevehicle.

As used in throughout this description, a “fully-learned” machinelearning model is a model that is trained to compute a desiredprediction. In other words, a fully-learned model generates an outputbased solely on being trained on training data rather than onhuman-programmed decisions.

FIG. 1 is a diagram of an example system 100. The system 100 includes atraining system 110 and an on-board system 130.

The on-board system 130 is physically located on-board a vehicle 122.The vehicle 122 in FIG. 1 is illustrated as an automobile, but theon-board system 130 can be located on-board any appropriate vehicletype. The vehicle 122 can be a fully autonomous vehicle that uses objectdetection predictions to inform fully-autonomous driving decisions. Thevehicle 122 can also be a semi-autonomous vehicle that uses objectdetection predictions to aid a human driver. For example, the vehicle122 can autonomously apply the brakes if a full-vehicle predictionindicates that a human driver is about to collide with a detectedobject, e.g., a pedestrian, a cyclist, another vehicle.

The on-board system 130 includes one or more sensor subsystems 132. Thesensor subsystems include a combination of components that receivereflections of electromagnetic radiation, e.g., LIDAR systems thatdetect reflections of laser light, radar systems that detect reflectionsof radio waves, and camera systems that detect reflections of visiblelight.

The sensor subsystems can also include combinations of short-range andlong-range laser sensors. For example, a short-range laser sensor can beused to detect the ground surrounding vehicle 122 and nearby objectswithin 40 meters from the vehicle 122. In another example, a long-rangelaser sensor can be used to detect objects up to 80 meters around thevehicle 122.

The raw input sensor data indicates a distance, a direction, and anintensity of reflected radiation. Each sensor can transmit one or morepulses of electromagnetic radiation in a particular direction and canmeasure the intensity of any reflections as well as the time that thereflection was received. A distance can be computed by determining howlong it took between a pulse and its corresponding reflection. Eachsensor can continually sweep a particular space in angle, azimuth, orboth. Sweeping in azimuth, for example, can allow a sensor to detectmultiple objects along a same line of sight.

The sensor subsystems 132 provide input sensor data 155 to an on-boardneural network subsystem 134. The input sensor data 155 can includemultiple channels of data, where each channel represents a differentcharacteristic of reflected electromagnetic radiation. Thus, multiplechannels of input sensor data 155 can be generated from measurementsfrom the same sensor.

The sensor subsystems 132, the on-board neural network subsystem 134, orsome combination of both, transform raw sensor data into the multiplechannels of input sensor data 155. To do so, the on-board system 130 canproject the various characteristics of the raw sensor data into a commoncoordinate system. The various characteristics of the raw sensor data,and their respective representations, will be discussed in more detailbelow with reference to FIG. 3.

The on-board neural network subsystem 134 implements the operations ofeach layer of a set of neural networks that are trained to makepredictions related to object detection, i.e., related to detectingobjects in the environment surrounding the vehicle. Thus, the on-boardneural network subsystem 134 includes one or more computing deviceshaving software or hardware modules that implement the respectiveoperations of each layer of the neural networks according to anarchitecture of the neural networks. The object detection neuralnetworks are described in more detail below with reference to FIGS. 2-4.

The on-board neural network subsystem 134 can implement the operationsof each layer of a neural network by loading a collection of modelparameters 172 that are received from the training system 110. Althoughillustrated as being logically separated, the model parameters 170 andthe software or hardware modules performing the operations may actuallybe located on the same computing device or, in the case of an executingsoftware module, stored within the same memory device.

The on-board neural network subsystem 134 can use hardware accelerationor other special-purpose computing devices to implement the operationsof one or more layers of the neural networks. For example, someoperations of some layers may be performed by highly parallelizedhardware, e.g., by a graphics processing unit or of another kind ofspecialized computing device. In other words, not all operations of eachlayer need to be performed by central processing units (CPUs) of theon-board neural network subsystem 134.

The on-board neural network subsystem 134 uses the input sensor data 155to generate one or more object detection predictions 165. The on-boardneural network subsystem 134 can provide the one or more objectdetection predictions 165 to a planning subsystem 136, a user interfacesubsystem 138, or both.

When a planning subsystem 136 receives the one or more object detectionpredictions 165, the planning subsystem 136 can use the one or moreobject detection predictions 165 to make fully-autonomous orsemi-autonomous driving decisions. For example, the planning subsystem136 can generate a fully-autonomous plan to navigate through or aroundother vehicles on a highway while also avoiding cyclists andpedestrians. As another example, the planning subsystem 136 can generatea semi-autonomous recommendation for a human driver to apply the brakes.

A user interface subsystem 138 can receive the object detectionpredictions 165 and can generate a user interface presentation thatindicates the locations of nearby objects. For example, the userinterface subsystem 138 can generate a user interface presentationhaving image or video data containing a representation of the regions ofspace that are likely to be occupied by objects. An on-board displaydevice can then display the user interface presentation for passengersof the vehicle 122.

The on-board neural network subsystem 134 can also use the input sensordata 155 to generate training data 123. The training data 123 caninclude the projected representations of the different channels of inputsensor data. The on-board system 130 can provide the training data 123to the training system 110 in offline batches or in an online fashion,e.g., continually whenever it is generated.

The training system 110 is typically hosted within a data center 112,which can be a distributed computing system having hundreds or thousandsof computers in one or more locations.

The training system 110 includes a training neural network subsystem 114that can implement the operations of each layer of a neural network thatis designed to make object detection predictions from input sensor data.The training neural network subsystem 114 includes a plurality ofcomputing devices having software or hardware modules that implement therespective operations of each layer of the neural network according toan architecture of the neural network.

The training neural networks generally have the same architecture as theon-board neural networks. However, the training system 110 need not usethe same hardware to compute the operations of each layer. In otherwords, the training system 110 can use CPUs only, highly parallelizedhardware, or some combination of these.

The training neural network subsystem 114 can compute the operations ofeach layer of the neural network using current values of parameters 115stored in a collection of model parameters 170. Although illustrated asbeing logically separated, the model parameters 170 and the software orhardware modules performing the operations may actually be located onthe same computing device or on the same memory device.

The neural network subsystem 114 can receive training examples 123 asinput. The training examples 123 can include auto-labeled training data,human-labeled training data, or some combination of the two. Each of thetraining examples 123 includes a representation of the differentchannels of input sensor data as well as one or more labels thatindicate the location of objects within regions of space represented bythe input sensor data. For example, the training examples 123 caninclude input sensor data for reference objects that are predeterminedto be associated with different object categories, e.g., pedestrians,cyclists. In some implementations, training examples 123 can includemultiple objects for each object category.

The training neural network subsystem 114 can generate, for eachtraining example 123, one or more object detection predictions 135. Atraining engine 116 analyzes the object detection predictions 135 andcompares the object detection predictions to the labels in the trainingexamples 123. The training engine 116 then generates updated modelparameter values 145 by using an appropriate updating technique, e.g.,backpropagation. The training engine 116 can then update the collectionof model parameters 170 using the updated model parameter values 145.

After training is complete, the training system 110 can provide a finalset of model parameter values 171 to the on-board system 130 for use inmaking fully autonomous or semi-autonomous driving decisions. Thetraining system 110 can provide the final set of model parameter values171 by a wired or wireless connection to the on-board system 130.

FIG. 2 is a flow chart of an example of a process 200 for performinghigh precision object detection for an environment in proximity of anautonomous vehicle. For convenience, the process 200 will be describedas being performed by a system of one or more computers located in oneor more locations. For example, a neural network system, e.g., theon-board system 130 of FIG. 1, appropriately programmed in accordancewith this specification, can perform the process 200.

In general, the neural network system performs the objection detectiontechniques described throughout using a two-stage neural networkarchitecture depicted in more detail in FIG. 3A. During an initial “highrecall” stage, the system processes raw sensor data to generate datathat the system then processes in a “high precision” stage. The systemperforms the operations of the process 200 within the high precisionstage based on input that represents the output of the high recallstage.

Briefly, the process 200 can include obtaining sensor data includinglaser projections and a camera image (210), processing the laserprojections and the camera image (220), obtaining an inputcharacterizing a first region of an environment (230), generatingalternate representations of a projected laser image, a camera imagepatch, and a feature vector (240), processing the alternaterepresentations of the projected laser image, the camera image patch,and the feature vector through a combining sub-neural network (250), andgenerating a respective score for each object category in a set of oneor more object categories (260).

In more detail, the system obtains sensor data including laserprojections and a camera image (210). For instance, as illustrated inFIG. 3A, sensor data 301 includes laser projections generated from athree-dimensional laser sensor reading of a first region of anenvironment surrounding an autonomous vehicle. The sensor data 301 alsoincludes a camera image of the first region.

The system processes the laser projections and the camera image (220).For example, as illustrated in FIG. 3A, a high-recall neural network305A processes the laser projections to generate a projected laser image302A. FIG. 4A illustrates the process for processing the laserprojections to generate the projected laser image 302A. In addition, ahigh-recall neural network 305B processes the camera image to generate acamera image patch 302B. The projected laser image 302A, the cameraimage input 302B, and an associated feature vector 302C are thenprovided as input 302 to the high-precision object neural network 310.

As described above, in some implementations, the input 302 is generatedbased on the output of one or more of multiple high-recall objectdetection neural networks, which process raw sensor data 301 collectedby the laser and camera sensors of an autonomous vehicle. Each of themultiple high-recall object detection neural networks are trained toprocess a certain type of sensor data collected by the sensor subsystemof an autonomous vehicle.

For example, as illustrated in FIG. 4A, a first high-recall objectdetection neural network 305A can be used to process a projected laserimage 404 generated from a set of top-down laser projections 402A-D. Thehigh-recall object detection neural network 305A can be used to identifycandidate regions within the projected laser image 404 that arepredicted to include objects. In another example, as illustrated in FIG.4B, a high-recall object detection neural network 305B processes acamera image 452. The high-recall object detection neural network 305Bcan be used to generate camera image patches corresponding to objectsdetected within the environment represented within the camera image 452.

The candidate regions within the projected laser image and the camerathat are identified by the high-recall object detection neural networks305A and 305B, respectively, can then be used to generate the input 302.In some implementations, the candidate regions are used to extractportions of the projected laser image and the camera image that aredetermined to have a high likelihood of including an object of interest,and discard other portions of the projected laser image and the cameraimage patch. Thus, as described in more detail in FIG. 3A, the output ofthe high-recall object detection neural networks 305A and 305B are usedto generate the input that is processed by the system.

In some implementations, the sensor data 301 includes both long-rangelaser data collected by a long-range laser sensor and short-range laserdata collected by a short-range laser sensor. For example, the sensorsubsystem of the autonomous vehicle can include both long-range andshort-range laser sensors that capture laser data from short-rangeregions (e.g., less than 40 meters from the vehicle) and long-rangeregions (e.g., up to 80 meters from the vehicle). In suchimplementations, the long-range and short-range sensor data areprocessed using two separate high precision object detection neuralnetworks, e.g., a long-range high precision object detection neuralnetwork and a short-range high precision object detection neuralnetwork. An example of a neural network architecture for suchimplementation is discussed below with respect to FIG. 3B.

In some implementations, the architecture 300A receives sensor data 301that includes both long-range laser sensor data and short-range lasersensor data, but only one high precision object detection neural networkis used to process short-range laser sensor data only. In suchimplementations, the long-range sensor data is used only to generate theinput components of the input 302, but not processed by the highprecision object detection neural network 310.

The system obtains an input characterizing a first region of anenvironment (230). For instance, as illustrated in FIG. 3A, the input302 can include the projected laser image 302A, a camera image patchDetails relating to the projected laser image, the camera image patch,and the feature vector are described below in FIGS. 3A, 3B, 4A, and 4B.

The system processes the input using a high precision object detectionneural network (240). As described above, the input processed by usingthe high precision object detection neural network is generated based onthe output of multiple high-recall object detection neural networks,e.g., the high-recall object detection neural networks 305A and 305B.The components of the input, e.g., the projected laser image, the cameraimage patch, and the feature vector, are then processed using a highprecision object detection neural network.

As illustrated in FIG. 3A, the high precision object detection neuralnetwork includes multiple sub-neural networks that are each trained toprocess a respective input component. For example, a high precisionobject detection neural network can include a laser sub-neural network310A that processes a projected laser image 302A, a camera sub-neuralnetwork 310B that processes a camera image patch 302B, and a featuresub-neural network 310C that processes a feature vector 302C.

The feature vector 302C encodes context information, e.g., distancebetween the autonomous vehicle and the road edge, that characterizes theenvironment. In some instances, the feature vector 302C encodes contextinformation that is captured independently from the data captured bylaser sensors and the camera sensors. For example, the feature vector302C can include context information that is obtained using datacollected by other sensors and/or data obtained from externalinformation sources.

As described in more detail below, the sub-neural networks 310A, 310B,and 310C each process a corresponding input component to generatealternative representations of the corresponding input component. Thealternative representations are tensors, i.e., one-dimensional ormulti-dimensional arrays, that represent the corresponding input. Forexample, the alternative representations may have predetermineddimensions that are associated with the corresponding input.

The system processes the alternate representations of the projectedlaser image, the camera image patch, and the feature vector through acombining sub-neural network (250). For example, as illustrated in FIG.3A, the alternative representations 304A, 304B, and 304C are provided asinput to a combining sub-neural network 310D. The combining sub-neuralnetwork 310D can include one or more visual combining neural networklayers to generate visual combined representations of the processedproject laser image and the processed the camera image patch. The visualcombined representation and the alternative representation of thefeature vector are then processed by one or more final combining layersto generate a final combined representation, which is used to generateobject scores for objects detected within the visual combinedrepresentation.

The system generates a respective score for each object category in aset of one or more object categories from the alternativerepresentations (260). The high precision object detection neuralnetwork may compute a respective object score for each object categoryin a set of one or more object categories. Each object score representsa respective likelihood that an object belonging to the object categoryis located in the first region of the environment. For example, anobject score can be computed for each object category. The object scorefor a particular object category represents a likelihood that an objectin a particular object category is detected within the projected laserimage and/or the camera image patch. Detailed descriptions related tocomputing object scores are provided in FIGS. 4A and 4B, respectively.

FIG. 3A is a schematic diagram of an example of an architecture 300A ofa high precision object detection neural network 310. In general, thehigh precision object detection neural network 310 receives input data302 that includes a projected laser image 302A, a camera image patch302B, and a feature vector 302C. The high precision object detectionneural network 310 processes the input data 302 using multiplesub-neural networks 310A, 310B, 310C, and 310C to generate output data308 that includes object scores for different object categories.

In more detail, raw sensor data 301 collected by laser sensors andcamera sensors are initially pre-processed and analyzed using multiplehigh-recall neural networks that identify objects of interest detectedwithin the sensor data 301. For instance, as described in more detailbelow in FIGS. 4A and 4B, the output of the high-recall neural networksidentifies candidate regions that are predicted to include detectedobjects. The candidate regions are then used to generate the projectedlaser image 302A and the camera image patch 302B that are includedwithin the input 302.

For example, as illustrated in FIG. 4A, a set of top-down laserprojections generated from data collected by laser sensors arepre-processed and then provided as input to a high-recall laser neuralnetwork 305A in order to generate the projected laser image 302A. Inaddition, as illustrated in FIG. 4B, a camera image is similarlyprovided as input to a high-recall camera neural network 305B in orderto generate the camera image patch 302B. In some implementations, thelaser and camera data may include objects detected in both long-rangeand short-range regions. In such implementations, multiplehigh-precision object detection neural networks can be used to processdata for objects detected in the long-range and the short-range regionsas illustrated in FIG. 3B.

The projected laser image 302A, the camera image patch 302B, and thefeature vector 302C are provided as respective input to a lasersub-neural network 310A, a camera sub-neural network 310B, and a featuresub-neural network 310C. The laser sub-neural network 310A, the camerasub-neural network 310B, and the feature sub-neural network 310C thenprovide alternative representations of the projected laser image 302A,the camera image patch 302B, and the feature vector 302C as output tothe combining sub-neural network 310D. As described above, thealternative representations 304A, 304B, and 304C are tensors, i.e., asingle-dimensional or multi-dimensional array, with predetermineddimensions that represent the corresponding input associated with theprocessed projected laser image 302A, the camera image patch 302B, andthe feature vector 302C.

The laser sub-neural network 310A and the camera sub-neural network 310Bboth include stacks of convolutional neural network layers and,optionally, one or more other kinds of neural network layers, e.g.,pooling layers, normalization layers, batch normalization layers, amongothers. The feature sub-neural network 310C is a stack offully-connected layers that process the feature vector 302C to generatethe alternative representation 304C.

Once generated, the alternative representations 304A, 304B, and 304C areprovided as input to a combining sub-neural network 310D, which thengenerates the output data 308 including object scores for each categoryof objects. The combining sub-neural network 310D includes multipleneural network layers for combining the alternative representations304A, 304B, and 304C.

In some implementations, the combining sub-neural network 310D mayinclude one or more visual combining neural network layers to generate avisual combined representation of the alternative representation 304Aand the alternative representation 304B. In particular, in someimplementations, the visual combining layers include a depthconcatenation layer followed by one or more fully-connected layers. Adepth concatenation layer takes as input two or more tensors, e.g., thealternative representations of the alternative representation 304A andthe alternative representation 304B and concatenates the input tensorsin the depth dimension. This operation can be described as “stacking”the input tensors to generate a new tensor

The combining sub-neural network 310D may also include one or more finalcombining layers to generate a final combined representation based onprocessing the visual combined presentation and the alternativerepresentation 304C. The final combining layers may include a depthconcatenation layer followed by one or more fully connected layers.

The combining sub-neural network 310D then processes the final combinedrepresentation can through an output neural network layer to generatethe object scores that are included within the output data 308.

FIG. 3B is a schematic diagram of an example of an architecture 300B ofa high precision object detection neural network that processeslong-range and short-range sensor data. Compared to the architecture300A illustrated in FIG. 3A, which processes either long-range orshort-range data only, the architecture 300B can optionally be used inimplementations where the sensor subsystem of an autonomous vehicleincludes both long-range and short-range laser sensors.

In such implementations, the sensor data 301A can include short-rangelaser projections, long-range laser projections, and a camera image. Theshort-range and long-range laser projections can be pre-processed andprovided as input to the high-recall laser neural network 305A asdescribed above with respect to FIG. 3A. The camera image is alsosimilarly processed using the high-recall camera neural network 305Bdescribed above.

The output of the high-recall object detection neural networks 305A and305B are then separated based on the region of the environment where anobject is detected and for processing using a correspondinghigh-precision object detection neural network. This technique isoptionally used if objects are detected within both the long-rangeregion and the short-range region of the environment that is capturedwithin the camera image. For instance, if objects are only detectedwithin the short-range region of the environment within the cameraimage, then only the high precision object detection neural network 360Ais used to process an input that includes a short-range projected laserimage, a camera image patch of the detected object in the short-rangeregion, and a feature vector in a manner similar to that described abovein FIG. 3A. Alternatively, if only objects are detected within thelong-range region of the environment within the camera image, then onlythe high precision object detection neural network 360B is used toprocess an input that includes a long-range projected laser image, acamera image patch of the detected object in the long-range region, anda feature vector in a similar manner to that described above in FIG. 3B.

In implementations where objects are detected in both the short-rangeand the long-range region of the environment within the camera image,the system separates the output of the high-recall neural networks 305Aand 305B. Thus, short-range projected laser images, camera image patchesfor objects detected in the short-range region of the environment, andfeature vectors are provided as input to the high precision objectdetection neural network 350A, whereas long-range projected laserimages, camera image patches for objects detected in the long-region ofthe environment, and feature vectors are provided as input to the objectdetection neural network 360A. The output of the high precision objectdetection neural networks 360A and 360B are then combined to generatethe output data 308 as described above in FIG. 3A.

FIG. 4A is a schematic diagram of an example of processing a projectedlaser image 404 of an environment to generate bounding boxes thatrepresent detected objects in the projected laser image. A high-recallobject detection neural network 305A305A that is trained to detectobjects within a laser project image 404 is used to generate a processedlaser projected image 406 that includes bounding boxes for candidateregions of the laser projected image 404 that are predicted to includeobjects from specified object categories.

In general, the projected laser image 404 can be generated by combiningtop-down projections 402A, 402B, 402C, and 402D for different channelsof laser input data. The same or similar projections can be used forother kinds of electromagnetic radiation. Each of the top-downprojections 402A-D of a different channel of input sensor data onto aplane that is substantially parallel to the road on which the vehicle istraveling. The vehicle recording the input data is thus locatedsubstantially in the middle of each image.

Various types of laser sensors can be used to capture the top-down laserprojections described above. In some implementations, a short-rangelaser sensor can be used to collect sensor data up to 40 meters aroundthe vehicle. For example, a laser sensor can be used to detect theground surrounding the autonomous vehicle and nearby objects. In otherimplementations, a long-range laser senor can be used to collect sensordata up to 80 meters around the vehicle. For example, a long-range lasersensor can be used to detect long-range objects that are not detectableby the short-range laser sensors.

In some implementations, a combination of laser sensors (e.g.,short-range and long-range sensors) can be used in conjunction tosimultaneously detect both long-range and short-range data.

As illustrated, a first top-down projection 402A is a projection of aheight channel that indicates a distance from the vehicle for eachtallest object. The intensities of the top-down projection are, however,all uniform. When tall objects are near the vehicle, the top-downprojection of the height channel data includes large regions of emptyspace. In addition, the top-down projection of the height channel datamay include regions in which no data was recorded. In the top-downprojection, the concentric circles and parallel lines represent regionsof space that were swept by a laser, but which generated no reflections.The system can include these visual features only in user-interfacepresentations for clarity, and need used them for making full-vehiclepredictions.

A second top-down projection 402B is a projection of an intensitychannel. The top-down projection indicates multiple objects for somelines of sight around the vehicle, and for each object, a distance and arelative intensity.

A third top-down projection 402C is a projection of a second returnchannel. The top-down projection indicates a distance from the vehicleof second returns for each direction around the vehicle. In thisexample, the second returns are likely to be car exhaust.

A fourth top-down projection 402D is a projection of an elongationchannel. The top-down projection indicates lines along which manyreflections were received, which are likely to be due to vegetation.

Once the projected laser image 404 is generated based on combining theindividual top-down laser projections discussed above, the projectedlaser image 404 is provided as input to the high-recall object detectionneural network 305A. The high-recall object detection neural network305A is trained to identify regions within the projected laser image 404that are likely to include an object associated with an object category.As described above, examples of object categories include pedestrians,cyclists, or other vehicles near the vicinity of the vehicle as ittravels on a road.

In some implementations, the top-down projections, or the projectedlaser image 404, can be processed prior to being sent as input to thehigh-recall object detection neural network 305A. For example, theprojected laser image 404 can be normalized using different techniquesbased on the features to be analyzed by the high-recall object detectionneural network 305A.

The high-recall object detection neural network 305A is trained toidentify candidate regions within the projected laser image 404 that arepredicted to include specific objects of interest using regressiontechniques. The high-recall object detection neural network 305A thengenerates bounding boxes that specify the boundaries of the candidateregions. The classification techniques used by the high-recall objectdetection neural network 305A enable the generation of bounding boxes indifferent regions of the projected laser image, and with differentshapes (e.g., width, height, rotation angle).

The high-recall object detection neural network 305A also computesconfidence scores for each detected object within the projected laserimage 404. The value of a respective confidence score reflects alikelihood that a detected object belongs to an object category. In someinstances, the high-recall object detection neural network 305A iscapable of generating small bounding boxes based on limited sensor dataavailable for a particular region (e.g., a 3×3 pixel bounding box). Anexample of an architecture for the high-recall object detection neuralnetwork 305A is the “DeepMultiBox” deep neural network described byErhan et. al. in a research publication titled “Scalable ObjectDetection using Deep Neural Networks,.” available atttps://pdfs.semanticscholar.org/0674/792f5edac72b77fb1297572c15b153576418.pdf.

In some implementations where the laser sub-neural network 310A isconfigured to identify multiple object categories, the high-recallobject detection neural network 305A may compute respective confidencescores for each object category for a given detected object. In thisregard, the high-recall object detection neural network 305A may comparethe values of the respective confidence scores to classify a detectedobject from among multiple object categories.

In the example depicted in FIG. 4A, the high-recall object detectionneural network 305A identifies five bounding boxes within the processedlaser projected image 404 corresponding to detected objects labelled as“A,” “B,” “C,” “D,” and “E” in the figure. The high-recall objectdetection neural network 305A also computes confidence scores for eachobject for a “CYCLIST” object category, which are included in table 408.As depicted, high-recall object detection neural network 305A computesthe highest confidence score for objects “C” and “B,” indicating thatthese objects are most likely to represent a cyclist in the vicinity ofthe vehicle. In contrast, the high-recall object detection neuralnetwork 305A computes the lowest confidence score for object “D,”indicating that this object is either an object associated with adifferent object category, e.g., a pedestrian or a vehicle, a falselydetected object.

The regions of the projected laser image 406 that are determined toinclude the objects A-E are then correlated with objects detected withina camera image of the environment as described below with respect toFIG. 4B. For instance, corresponding regions in the camera image for theobjects A-E detected within the projected laser image 406 are identifiedand extracted. In this regard, the input 302 that is provided to thehigh precision object detection neural network 310 includescorresponding regions of the projected laser image and the camera imagepatch.

In addition, the regions of the projected laser image 406 that aredetermined to include the objects A-E are also used generate theprojected laser image 302A. For example, in some implementations, theregions of the projected laser image 406 that include objects A-E can beextracted whereas the other portions of the projected laser image 406can be discarded so that only the portions of the projected laser image406 are determined to have a high likelihood of including an object ofinterest are processed by the laser sub-neural network 310A.

FIG. 4B is a schematic diagram of an example of processing a cameraimage 452 of an environment to generate bounding boxes that representdetected objects in the camera image 452. As discussed above, ahigh-recall object detection neural network 305B that is trained todetect objects within a camera image 452 is used to generate a processedcamera image 454 that includes bounding boxes for candidate regions ofthe camera image 452 that are predicted to include one or more objectsfrom one or more specified object categories. The bounding boxes arethen used to identify regions of the camera image that include objectsdetected by the high-recall object detection neural network 305B.

In general, the camera image 452 is initially pre-processed to extract aset of raw camera image patches 452A. As described above, each cameraimage patch refers to a ,e.g., a randomly-selected region or a regionselected using other criteria by an external process, of the cameraimage 452 that is within the field of view of the camera capturing thecamera image 452. For example, each camera image patch represents acollection of pixels within the camera image 452 that are extractedbased on a particular patch size. In some instances, multiplecollections of camera image patches of different sizes are extractedfrom various regions of the camera image 452 (e.g., a collectionten-by-ten pixel patches and a collection of twenty-by-twenty pixelpatches) such that the entire field of view of the camera image 542 isrepresented within the collections of the image patches. . The set ofraw camera image patches 452A can be extracted using any suitable imageprocessing techniques and camera calibration information.

In the example depicted, two raw camera image patches A and B areextracted from the camera image 452, e.g., a camera image patch thatincludes a white vehicle nearby in the bottom right of the camera image452, and an another camera image patch that includes an individualexiting a parked truck in front of the white truck.

The set of raw camera image patches 452A are then provided as input tothe high-recall object detection neural network 305B. The high-recallobject detection neural network 305B identifies candidate regions withinthe camera image 452 (e.g., regions within the camera image patches)that are determined to include an object associated with one or moreobject categories. The high-recall object detection neural network 305Bis generally trained to identify and verify objects within the set ofrange-normalized camera image patches 452B that are provided as input.The detected objects and their computed confidence scores are includedin a table 456.

The camera image patches that are determined by the high-recall objectdetection neural network 305B are then processed to generate a set ofrange-normalized camera image patches. For instance, the candidateimages of the camera image patches are initially projected back into athree-dimensional coordinate system of the camera image 454.

For instance, the camera's field of view is initially calibrated toidentify a three-dimensional coordinate system that correlates a pointwithin the camera image 452 to a physical location within the region ofthe environment that is captured within the camera image 452. Forexample, the three-dimension coordinate system enables the camera, orany other post-processing device, to identify an object within thecamera image 452 and a corresponding range value representing a distancebetween the vehicle and the physical location of the object in theregion of the environment that is captured within the camera image 452.In another example, the three dimensional coordinate system enables thecamera or the post-processing device to identify a three-dimensionalcoordinate for a detected object, e.g., x, y, and z coordinatescorresponding to the object's physical location within the region of theenvironment captured in the camera image 452. These three-dimensionalcoordinates can then be correlated with a two-dimensional coordinatesrepresenting the locations of pixels associated with the object withinthe camera image 452.

The set of camera image patches that include objects are adjusted usingthe computed range values corresponding to predicted objects within araw camera image patch to generate a set of range-normalized cameraimage patches that each include adjusted camera image patchescorresponding to detected objects. As discussed above, a range value canbe used to represent a distance between the vehicle and a physicallocation of a given within the region of the environment that iscaptured within the camera image 452. Thus, the size of camera imagepatches for objects that are further away from the vehicle are smallerthan the size of camera image patches for objects that are closer to thevehicle.

The range normalized camera image patches are generated such that theobjects included within each of the camera image patches aresubstantially the same size. Thus, the raw camera image patches areresized such that the each of the camera image patches within the rangenormalized camera image patches are roughly the same size. This reducespotential object classification inaccuracies in the “high-precision”stage by the camera sub-neural network 310B as illustrated in FIG. 3A.

The locations of the bounding boxes within the environment of the cameraimage 454 can then be used to determine if a detected object is within ashort-range region of the environment or a long-range region of theenvironment. Thus, the location of the generated bounding box is used todetermine the corresponding projected laser image to provide as input toa high precision object detection neural network as described above. Forexample, as described above, the coordinate system associated with thecamera image can be used to identify a range value associated with thedetected object, which is then used to determine whether the object isin a long-range or a short-range region of the environment.

In some implementations, where the system collects both long-range andshort-range sensor data, the location of the bounding box can thereforebe used to determine whether to provide input to either the highprecision object detection neural network 360A or 360B as describedabove with respect to FIG. 3B. The location of the bounding boxes canalso be used to generate either a long-range projected laser image or ashort-range projected laser corresponding to the region containing thebounding box. For example, if a location of a bounding box indicatesthat the detected object is located within a short-range region of theenvironment, a corresponding short-range laser projection image may begenerated. Alternatively, if a location of a bounding box indicates thatthe detected objected is located within a long-range region of theenvironment, a corresponding long-range laser projection image may begenerated.

In the example depicted, of the two objects initially detected, thehigh-recall object detection neural network 305B only identifies abounding box within the processed camera image 454 for the camera imagepatch A of the individual exiting the parked truck, but not for thecamera image patch B of the white vehicle. In this example, thehigh-recall object detection neural network 305B is trained to identifyobjects that are a part of the object category “PEDESTRIAN.” Because theconfidence score computed for the camera patch A is lower than athreshold, the high-recall object detection neural network 305Bdisregards the white vehicle as not being included in the “PEDESTRIAN”class, and therefore only generates a bounding box for the individualexiting the truck as illustrated.

In some implementations, the high-recall object detection neural network305B may identify objects within the entire camera image 452 instead ofthe set of range-normalized camera image patches 452B as illustrated inthe figure. In such implementations, the camera image 452 may beprovided as input to the high-recall object detection neural network305B, which then identifies candidate regions with bounding boxes withinthe camera image 454 using similar objection recognition and detectiontechniques as described above. In addition, camera image patchescorresponding to bounding boxes may be extracted from the camera image454 and then provided as input 302B to the high precision objectdetection neural network 310 as illustrated in FIG. 3A.

The features described can be implemented in digital electroniccircuitry, or in computer hardware, firmware, software, or incombinations of them. The apparatus can be implemented in a computerprogram product tangibly embodied in an information carrier, e.g., in amachine-readable storage device for execution by a programmableprocessor; and method steps can be performed by a programmable processorexecuting a program of instructions to perform functions of thedescribed implementations by operating on input data and generatingoutput. The described features can be implemented advantageously in oneor more computer programs that are executable on a programmable systemincluding at least one programmable processor coupled to receive dataand instructions from, and to transmit data and instructions to, a datastorage system, at least one input device, and at least one outputdevice. A computer program is a set of instructions that can be used,directly or indirectly, in a computer to perform a certain activity orbring about a certain result. A computer program can be written in anyform of programming language, including compiled or interpretedlanguages, and it can be deployed in any form, including as astand-alone program or as a module, component, subroutine, or other unitsuitable for use in a computing environment.

Suitable processors for the execution of a program of instructionsinclude, by way of example, both general and special purposemicroprocessors, and the sole processor or one of multiple processors ofany kind of computer. Generally, a processor will receive instructionsand data from a read-only memory or a random access memory or both. Theessential elements of a computer are a processor for executinginstructions and one or more memories for storing instructions and data.Generally, a computer will also include, or be operatively coupled tocommunicate with, one or more mass storage devices for storing datafiles; such devices include magnetic disks, such as internal hard disksand removable disks; magneto-optical disks; and optical disks. Storagedevices suitable for tangibly embodying computer program instructionsand data include all forms of non-volatile memory, including by way ofexample semiconductor memory devices, such as EPROM, EEPROM, and flashmemory devices; magnetic disks such as internal hard disks and removabledisks; magneto-optical disks; and CD-ROM and DVD-ROM disks. Theprocessor and the memory can be supplemented by, or incorporated in,ASICs (application-specific integrated circuits).

To provide for interaction with a user, the features can be implementedon a computer having a display device such as a CRT (cathode ray tube)or LCD (liquid crystal display) monitor for displaying information tothe user and a keyboard and a pointing device such as a mouse or atrackball by which the user can provide input to the computer.Additionally, such activities can be implemented via touchscreenflat-panel displays and other appropriate mechanisms.

The features can be implemented in a computer system that includes aback-end component, such as a data server, or that includes a middlewarecomponent, such as an application server or an Internet server, or thatincludes a front-end component, such as a client computer having agraphical user interface or an Internet browser, or any combination ofthem. The components of the system can be connected by any form ormedium of digital data communication such as a communication network.Examples of communication networks include a local area network (“LAN”),a wide area network (“WAN”), peer-to-peer networks (having ad-hoc orstatic members), grid computing infrastructures, and the Internet.

The computer system can include clients and servers. A client and serverare generally remote from each other and typically interact through anetwork, such as the described one. The relationship of client andserver arises by virtue of computer programs running on the respectivecomputers and having a client-server relationship to each other.

While this specification contains many specific implementation details,these should not be construed as limitations on the scope of anyinventions or of what may be claimed, but rather as descriptions offeatures specific to particular implementations of particularinventions. Certain features that are described in this specification inthe context of separate implementations can also be implemented incombination in a single implementation. Conversely, various featuresthat are described in the context of a single implementation can also beimplemented in multiple implementations separately or in any suitablesub-combination. Moreover, although features may be described above asacting in certain combinations and even initially claimed as such, oneor more features from a claimed combination can in some cases be excisedfrom the combination, and the claimed combination may be directed to asub-combination or variation of a sub-combination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. In certain circumstances, multitasking and parallel processingmay be advantageous. Moreover, the separation of various systemcomponents in the implementations described above should not beunderstood as requiring such separation in all implementations, and itshould be understood that the described program components and systemscan generally be integrated together in a single software product orpackaged into multiple software products.

Thus, particular implementations of the subject matter have beendescribed. Other implementations are within the scope of the followingclaims. In some cases, the actions recited in the claims can beperformed in a different order and still achieve desirable results. Inaddition, the processes depicted in the accompanying figures do notnecessarily require the particular order shown, or sequential order, toachieve desirable results. In certain implementations, multitasking andparallel processing may be advantageous.

While this specification contains many specific implementation details,these should not be construed as limitations on the scope of anyinvention or on the scope of what may be claimed, but rather asdescriptions of features that may be specific to particular embodimentsof particular inventions. Certain features that are described in thisspecification in the context of separate embodiments can also beimplemented in combination in a single embodiment. Conversely, variousfeatures that are described in the context of a single embodiment canalso be implemented in multiple embodiments separately or in anysuitable sub-combination. Moreover, although features may be describedabove as acting in certain combinations and even initially be claimed assuch, one or more features from a claimed combination can in some casesbe excised from the combination, and the claimed combination may bedirected to a sub-combination or variation of a sub-combination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. In certain circumstances, multitasking and parallel processingmay be advantageous. Moreover, the separation of various system modulesand components in the embodiments described above should not beunderstood as requiring such separation in all embodiments, and itshould be understood that the described program components and systemscan generally be integrated together in a single software product orpackaged into multiple software products.

Particular embodiments of the subject matter have been described. Otherembodiments are within the scope of the following claims. For example,the actions recited in the claims can be performed in a different orderand still achieve desirable results. As one example, the processesdepicted in the accompanying figures do not necessarily require theparticular order shown, or sequential order, to achieve desirableresults. In certain some cases, multitasking and parallel processing maybe advantageous.

The invention claimed is:
 1. A method comprising: processing, by one ormore computers, a camera image collected by one or more cameras of avehicle using a first high-recall object detection neural network,wherein the camera image depicts an environment in a vicinity of thevehicle, and wherein the first high-recall object detection neuralnetwork is configured to: receive the camera image; and process thecamera image to generate: (i) data defining a plurality of boundingboxes in the camera image, each bounding box showing a different regionof the environment; and (ii) for each of the plurality of boundingboxes, a respective first confidence score that represents a probabilitythat an object belonging to an object category from a second set of oneor more object categories is present in the region of the environmentthat shows the bounding box; determining, based at least on therespective first confidence scores, that additional processing isrequired to determine a higher-precision confidence score for a firstregion of the environment shown by a first bounding box of the pluralityof bounding boxes; in response to determining that additional processingis required: obtaining, by the one or more computers, an inputcharacterizing the first region of an environment, the input comprising:(i) a projected laser image collected by one or more laser sensors ofthe vehicle and generated from a three-dimensional laser sensor readingof the first region of the environment; (ii) a camera image patchgenerated from the camera image of the environment and corresponding tothe first bounding box for the first region; and (iii) a feature vectorof features characterizing the first region; and processing, by the oneor more computers, the input using a high precision object detectionneural network to generate a respective object score for each objectcategory in a first set of one or more object categories, wherein eachobject score represents a respective probability that an objectbelonging to the object category is located in the first region of theenvironment, and wherein processing the input comprises: processing theprojected laser image through a laser sub-neural network to generate analternative representation of the projected laser image; processing thecamera image patch through a camera sub-neural network to generate analternative representation of the camera image patch; processing thefeature vector through a feature sub-neural network to generate analternative representation of the feature vector; and processing, by theone or more computers, the alternative representation of the projectedlaser image, the alternative representation of the camera image patch,and the alternative representation of the feature vector through acombining sub-neural network to generate the respective object score foreach of the one or more object categories.
 2. The method of claim 1,wherein the environment is an environment in proximity of an autonomousvehicle, and wherein the three-dimensional laser sensor reading and thecamera image are captured by sensors of the autonomous vehicle.
 3. Themethod of claim 1, wherein the combining sub-neural network isconfigured to: process the alternative representation of the laserprojected image and the alternative representation of the camera imagepatch through one or more visual combining neural network layers togenerate a visual combined representation; process the visual combinedrepresentation and the alternative representation of the feature vectorthrough one or more final combining layers to generate a final combinedrepresentation; and process the final combined representation through anoutput neural network layer to generate the object scores.
 4. The methodof claim 1, further comprising: generating a plurality of camera imagepatches from the obtained camera image of the environment; determining arespective size value for each of the plurality of camera image patches;adjusting the respective size values for each of the plurality of cameraimage patches to generate a plurality of range-normalized camera imagepatches, wherein the values of the respective sizes for each of theplurality of range-normalized camera image patches are substantiallysimilar; and processing the plurality of range-normalized camera imagepatches using the first high-recall object detection neural network. 5.The method of claim 1, further comprising: obtaining a projected laserimage of the environment; processing the projected laser image using asecond high-recall object detection neural network, wherein the secondhigh-recall object detection neural network is configured to: receivethe projected laser image; and process the projected laser image togenerate: (i) data defining a plurality of bounding boxes in theprojected laser image; and (ii) for each of the plurality of boundingboxes, a respective second confidence score that represents aprobability that an object belonging to an object category from thesecond set of one or more object categories is present in the region ofthe environment shown in the bounding box.
 6. The method of claim 5,further comprising: obtaining respective top-down projections of aplurality of channels of the input characterizing the first region ofthe environment, each top-down projection representing a projection ofan area surrounding the autonomous vehicle; and combining the respectivetop-down projections to generate the projected laser image.
 7. Themethod of claim 1, further comprising: determining, from the firstconfidence scores and the second confidence scores, that additionalprocessing is required to determine a higher-precision confidence scorefor the first region, and in response, obtaining the first inputcharacterizing the first region and processing the first input using thehigh precision object detection neural network.
 8. The method of claim1, wherein the projected laser image is generated from athree-dimensional laser sensor reading collected by a long-range lasersensor.
 9. The method of claim 1, wherein the projected laser image isgenerated from a three-dimensional laser sensor reading collected by ashort-range laser sensor.
 10. The method of claim 1, wherein the one ormore object categories comprise at least a pedestrian category and acyclist category.
 11. The method of claim 1, wherein: the obtained inputcharacterizing the first region of the environment comprises amultiplexed signal, and processing the input using a high precisionobject detection neural network comprises processing each subcomponentof the obtained input to generate a respective object score for aparticular object category, wherein each subcomponent of the obtainedinput corresponds to a different object category.
 12. A systemcomprising: one or more computers; and a non-transitorycomputer-readable medium coupled to the one or more computers havinginstructions stored thereon, which, when executed by the one or morecomputers, cause the one or more computers to perform operationscomprising: processing, by one or more computers, a camera imagecollected by one or more cameras of a vehicle using a first high-recallobject detection neural network, wherein the camera image depicts anenvironment in a vicinity of the vehicle, and wherein the firsthigh-recall object detection neural network is configured to: receivethe camera image; and process the camera image to generate: (i) datadefining a plurality of bounding boxes in the camera image, eachbounding box showing a different region of the environment; and (ii) foreach of the plurality of bounding boxes, a respective first confidencescore that represents a probability that an object belonging to anobject category from a second set of one or more object categories ispresent in the region of the environment that shows the bounding box;determining, based at least on the respective first confidence scores,that additional processing is required to determine a higher-precisionconfidence score for a first region of the environment shown by a firstbounding box of the plurality of bounding boxes; in response todetermining that additional processing is required: obtaining, by theone or more computers, an input characterizing the first region of anenvironment, the input comprising: (i) a projected laser image collectedby one or more laser sensors of the vehicle and generated from athree-dimensional laser sensor reading of the first region of theenvironment; (ii) a camera image patch generated from the camera imageof the environment and corresponding to the first bounding box for thefirst region; and (iii) a feature vector of features characterizing thefirst region; and processing, by the one or more computers, the inputusing a high precision object detection neural network to generate arespective object score for each object category in a first set of oneor more object categories, wherein each object score represents arespective probability that an object belonging to the object categoryis located in the first region of the environment, and whereinprocessing the input comprises: processing the projected laser imagethrough a laser sub-neural network to generate an alternativerepresentation of the projected laser image; processing the camera imagepatch through a camera sub-neural network to generate an alternativerepresentation of the camera image patch; processing the feature vectorthrough a feature sub-neural network to generate an alternativerepresentation of the feature vector; and processing, by the one or morecomputers, the alternative representation of the projected laser image,the alternative representation of the camera image patch, and thealternative representation of the feature vector through a combiningsub-neural network to generate the respective object score for each ofthe one or more object categories.
 13. The system of claim 12, whereinthe environment is an environment in proximity of an autonomous vehicle,and wherein the three-dimensional laser sensor reading and the cameraimage are captured by sensors of the autonomous vehicle.
 14. The systemof claim 12, wherein the combining sub-neural network is configured to:process the alternative representation of the laser projected image andthe alternative representation of the camera image patch through one ormore visual combining neural network layers to generate a visualcombined representation; process the visual combined representation andthe alternative representation of the feature vector through one or morefinal combining layers to generate a final combined representation; andprocess the final combined representation through an output neuralnetwork layer to generate the object scores.
 15. A non-transitorycomputer storage device encoded with a computer program, the programcomprising instructions that when executed by one or more computerscause the one or more computers to perform operations comprising:processing, by one or more computers, a camera image collected by one ormore cameras of a vehicle using a first high-recall object detectionneural network, wherein the camera image depicts an environment in avicinity of the vehicle, and wherein the first high-recall objectdetection neural network is configured to: receive the camera image; andprocess the camera image to generate: (i) data defining a plurality ofbounding boxes in the camera image, each bounding box showing adifferent region of the environment; and (ii) for each of the pluralityof bounding boxes, a respective first confidence score that represents aprobability that an object belonging to an object category from a secondset of one or more object categories is present in the region of theenvironment that shows the bounding box; determining, based at least onthe respective first confidence scores, that additional processing isrequired to determine a higher-precision confidence score for a firstregion of the environment shown by a first bounding box of the pluralityof bounding boxes; in response to determining that additional processingis required: obtaining, by the one or more computers, an inputcharacterizing the first region of an environment, the input comprising:(i) a projected laser image collected by one or more laser sensors ofthe vehicle and generated from a three-dimensional laser sensor readingof the first region of the environment; (ii) a camera image patchgenerated from the camera image of the environment and corresponding tothe first bounding box for the first region; and (iii) a feature vectorof features characterizing the first region; and processing, by the oneor more computers, the input using a high precision object detectionneural network to generate a respective object score for each objectcategory in a first set of one or more object categories, wherein eachobject score represents a respective probability that an objectbelonging to the object category is located in the first region of theenvironment, and wherein processing the input comprises: processing theprojected laser image through a laser sub-neural network to generate analternative representation of the projected laser image; processing thecamera image patch through a camera sub-neural network to generate analternative representation of the camera image patch; processing thefeature vector through a feature sub-neural network to generate analternative representation of the feature vector; and processing, by theone or more computers, the alternative representation of the projectedlaser image, the alternative representation of the camera image patch,and the alternative representation of the feature vector through acombining sub-neural network to generate the respective object score foreach of the one or more object categories.
 16. The non-transitorycomputer storage device of claim 15, wherein the environment is anenvironment in proximity of an autonomous vehicle, and wherein thethree-dimensional laser sensor reading and the camera image are capturedby sensors of the autonomous vehicle.
 17. The non-transitory computerstorage device of claim 15, wherein the combining sub-neural network isconfigured to: process the alternative representation of the laserprojected image and the alternative representation of the camera imagepatch through one or more visual combining neural network layers togenerate a visual combined representation; process the visual combinedrepresentation and the alternative representation of the feature vectorthrough one or more final combining layers to generate a final combinedrepresentation; and process the final combined representation through anoutput neural network layer to generate the object scores.